Lamellar Crystal Orientations Biased by Crystallization Kinetics in Polymer Thin Films

Quantifying and Modeling the Crystallinity of Polymers Confined in Nanopores

We propose a methodology to characterize the crystalline content of interfacial polymer layers in systems confined at the nanoscale level in a 2D geometry. Based on the crystallinity data of a set of polymers, we introduce a simple model to describe the gradient in crystallinity introduced by confining polymer chains in nanopores. Our model underscores the pivotal role that interfaces play in crystallization and unequivocally contradicts the existence of interfacial "dead" layers where crystallization cannot take place. Further, we verified that the organization of crystals near the pore walls resembles the macromolecular architecture of adsorbed layers, hinting at a strong interplay between crystallization and adsorption.

Confined Crystallization of Thin Plasma-Polymerized Nanocomposite Films with Maleic Anhydride and Cellulose Nanocrystals under Hydrolysis

The creation of novel surface morphologies through thin-film patterning is important from a scientific and technological viewpoint in order to control specific surface properties. The pulsed-plasma polymerization of thin nanocomposite films, including maleic anhydride (MA) and cellulose nanocrystals (CNC), may result in different metastable film morphologies that are difficult to control. Alternatively, the transformation of deposited plasma films into crystalline structures introduces unique and more stable morphologies. In this study, the structural rearrangements of plasma-polymerized (MA+CNC) nanocomposite films after controlled hydrolysis in a humid atmosphere were studied, including effects of plasma conditions (low duty cycle, variable power) and monomer composition (ratio MA/CNC) on hydrolysis stability. The progressive growth of crystalline structures with fractal dendrites was observed in confined thin films of 30 to 50 nm. The structures particularly formed on hydrophilic substrates and were not observed before on the more hydrophobic substrates, as they exist as a result of water penetration and interactions at the film/substrate interface. Furthermore, the nucleating effect and local pinning of the crystallites to the substrate near CNC positions enhanced the film stability. The chemical structures after hydrolysis were further examined through XPS, indicating esterification between the MA carboxylic acid groups and CNC surface. The hydrolysis kinetics were quantified from the conversion of anhydride groups into carboxylic moieties by FTIR analysis, indicating enhanced hydrolytic stability of p(MA+CNC) nanocomposite films relative to the pure p(MA) films.

Local orientation of chains at crystal/amorphous interfaces buried in isotactic polypropylene thin films

A better understanding of the aggregation states of polymer chains in thin films is of pivotal importance for developing thin film polymer devices in addition to its inherent scientific interest. Here we report the preferential orientation of the crystalline lamellae for isotactic polypropylene (iPP) in spin-coated films by grazing incidence of wide-angle X-ray diffraction in conjunction with sum frequency generation vibrational spectroscopy, which provides information on the local conformation of chains at crystal/amorphous interfaces buried in a thin film. The crystalline orientation of iPP, which formed cross-hatched lamellae induced by lamellar branching, altered from a mixture of edge-on and face-on mother lamellae to preferential face-on mother lamellae with decreasing thickness. The orientation of methyl groups at the crystal/amorphous interfaces in the interior region of the iPP films changed, accompanied by a change in the lamellar orientation.

Fabrication of PCDTBT Conductive Network via Phase Separation

Poly[N-9'-hepta-decanyl-2,7-carbazole-alt-5-5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (PCDTBT) is a stable semiconducting polymer with high rigidity in its molecular chains, which makes it difficult to organize into an ordered structure and affects the device performance. Here, a PCDTBT network consisting of aggregates and nanofibers in thin films was fabricated through the phase separation of mixed PCDTBT and polyethylene glycol (PEG). Using atomic force microscopy (AFM), the effect of the blending conditions (weight ratio, solution concentration, and molecular weight) and processing conditions (substrate temperature and solvent) on the resulting phase-separated morphologies of the blend films after a selective washing procedure was studied. It was found that the phase-separated structure's transition from an island to a continuous structure occurred when the weight ratio of PCDTBT/PEG changed from 2:8 to 7:3. Increasing the solution concentration from 0.1 to 3.0 wt% led to an increase in both the height of the PCDTBT aggregate and the width of the nanofiber. When the molecular weight of the PEG was increased, the film exhibited a larger PCDTBT aggregate size. Meanwhile, denser nanofibers were found in films prepared using PCDTBT with higher molecular weight. Furthermore, the electrical characteristics of the PCDTBT network were measured using conductive AFM. Our findings suggest that phase separation plays an important role in improving the molecular chain diffusion rate and fabricating the PCDTBT network.

Crystallization of Poly(ethylene)s with Regular Phosphoester Defects Studied at the Air-Water Interface

Poly(ethylene) (PE) is a commonly used semi-crystalline polymer which, due to the lack of polar groups in the repeating unit, is not able to form Langmuir or Langmuir-Blodgett (LB) films. This problem can be solved using PEs with hydrophilic groups arranged at regular distances within the polymer backbone. With acyclic diene metathesis (ADMET) polymerization, a tool for precise addition of polar groups after a certain interval of methylene sequence is available. In this study, we demonstrate the formation of Langmuir/LB films from two different PEs with regular phosphoester groups, acting as crystallization defects in the main chain. After spreading the polymers from chloroform solution on the water surface of a Langmuir trough and solvent evaporation, the surface pressure is recorded during compression under isothermal condition. These π-A isotherms, surface pressure π vs. mean area per repeat unit A, show a plateau zone at surface pressures of ~ (6 to 8) mN/m, attributed to the formation of crystalline domains of the PEs as confirmed by Brewster angle and epifluorescence microscopy. PE with ethoxy phosphoester defects (Ethoxy-PPE) forms circular shape domains, whereas Methyl-PPE-co-decadiene with methyl phosphoester defects and two different methylene sequences between the defects exhibits a film-like morphology. The domains/films are examined by atomic force microscopy after transferring them to a solid support. The thickness of the domains/films is found in the range from ~ (2.4 to 3.2) nm depending on the transfer pressure. A necessity of chain tilt in the crystalline domains is also confirmed. Grazing incidence X-ray scattering measurements in LB films show a single Bragg reflection at a scattering vector qxy position of ~ 15.1 nm-1 known from crystalline PE samples.

Impact of Nanoconfinement on Polylactide Crystallization and Gas Barrier Properties

The barrier properties of poly(l-lactide) (PLLA) were investigated in multinanolayer systems, probing the effect of confinement, the compatibility between the confining and the confined polymer, crystal orientation, and amorphous phase properties. The multilayer coextrusion process was used to confine PLLA between two amorphous polymers (polystyrene, PS; and polycarbonate, PC), which have different chemical affinities with PLLA. Confined PLLA layers of approximately 20 nm thickness were obtained. The multinanolayer materials were annealed at different temperatures to obtain PLLA crystallites with distinct polymorphs. PLLA annealed in PC/PLLA films at 120 °C afforded a crystallinity degree up to 65%, and PLLA annealed in PC/PLLA or PS/PLLA films at 85 °C had a crystallinity degree of 45%. WAXS measurements evidenced that the PLLA lamellas between PS layers had a mixed in-plane and on-edge orientation. PLLA lamellas between PC layers were uniquely oriented in-plane. DMA results evidenced a shift of the PC glass transition toward lower temperature, suggesting the possible presence of an interphase. The development of the rigid amorphous fraction (RAF) in the amorphous phase during annealing was impacted by the confiner polymer. The RAF content of semicrystalline PLLA was about 15% in PC/PLLA, whereas it was neglectable in PS/PLLA. The oxygen barrier properties appeared to be governed by RAF content, and no impact of the PLLA polymorph or the crystalline orientation was observed. This study shows that the confinement of PLLA on itself does not impact barrier properties but that the proper choice of the confiner polymer can lead to decrease the phase coupling which creates the RAF. It is the prevention of RAF that decreases permeability.

Tunable Properties of MAPLE-Deposited Thin Films in the Presence of Suppressed Segmental Dynamics

Processing polymer thin films by physical vapor deposition has been a major challenge due to material degradation. This challenge has limited our understanding of morphological control by top-down approaches that can be crucial for many applications. Recently, matrix-assisted pulsed laser evaporation (MAPLE) has emerged as an alternative route to fabricate polymer thin films from near-gas phase growth conditions. In this Letter, we investigate how this approach can result in a stable two-phase film structure of semicrystalline polymers via a unique combination of MAPLE and flash calorimetry. In the case of MAPLE-deposited poly(ethylene oxide) (PEO) thin films, we find a 35 °C enhancement in the glass transition temperature relative to melt-crystallized films, which is associated with irreversible chain adsorption in the amorphous region of the film. Remarkably, by varying substrate temperature during deposition, we reveal the ability to significantly tune the crystal orientation, extent of crystallinity, and lamellar thickness of MAPLE-deposited PEO thin films.

Crystal Growth in Polyethylene by Molecular Dynamics: The Crystal Edge and Lamellar Thickness

We carried out large-scale atomistic molecular dynamics simulations to study the growth of twin lamellar crystals of polyethylene initiated by small crystal seeds. By examining the size distribution of the stems-straight crystalline polymer segments-we show that the crystal edge has a parabolic profile. At the growth front, there is a layer of stems too short to be stable, and new stable stems are formed within this layer, leading to crystal growth. Away from the edge, the lengthening of the stems is limited by a lack of available slack length in the chains. This frustration can be relieved by mobile crystal defects that allow topological relaxation by traversing through the crystal. The results shed light on the process of polymer crystal growth and help explain initial thickness selection and lamellar thickening.

Crystalline and Spherulitic Morphology of Polymers Crystallized in Confined Systems

Due to the effects of microphase separation and physical dimensions, confinement widely exists in the multi-component polymer systems (e.g., polymer blends, copolymers) and the polymers having nanoscale dimensions, such as thin films and nanofibers. Semicrystalline polymers usually show different crystallization kinetics, crystalline structure and morphology from the bulk when they are confined in the nanoscale environments; this may dramatically influence the physical performances of the resulting materials. Therefore, investigations on the crystalline and spherulitic morphology of semicrystalline polymers in confined systems are essential from both scientific and technological viewpoints; significant progresses have been achieved in this field in recent years. In this article, we will review the recent research progresses on the crystalline and spherulitic morphology of polymers crystallized in the nanoscale confined environments. According to the types of confined systems, crystalline, spherulitic morphology and morphological evolution of semicrystalline polymers in the ultrathin films, miscible polymer blends and block copolymers will be summarized and reviewed.

Surface-Induced Frustration in Solid State Polymorphic Transition of Native Cellulose Nanocrystals

The presence of an interface generally influences crystallization of polymers from melt or from solution. Here, by contrast, we explore the effect of surface immobilization in a direct solid state polymorphic transition on individual cellulose nanocrystals (CNCs), extracted from a plant-based origin. The conversion from native cellulose I to cellulose III crystal occurred via a host-guest inclusion of ethylene diamine inside the crystal. A 60% reduction in CNC width (height) in atomic force microscopy images suggested that when immobilized on a flat modified silica surface, the stresses caused by the inclusion or the subsequent regeneration resulted in exfoliation, hypothetically, between the van der Waals bonded sheets within the crystal. Virtually no changes in dimensions were visible when the polymorphic transition was performed to nonimmobilized CNCs in bulk dispersion. With reservations and by acknowledging the obvious dissimilarities, the exfoliation of cellulose crystal sheets can be viewed as analogous to exfoliation of 2D structures like graphene from a van der Waals stacked solid. Here, the detachment is triggered by an inclusion of a guest molecule inside a host cellulose crystal and the stresses caused by the firm attachment of the CNC on a solid substrate, leading to detachment of molecular sheets or stacks of sheets.

Crystallization of ultrathin poly(3-hydroxybutyrate) films in blends with small amounts of poly(l-lactic acid): correlation between film thickness and molecular weight of poly(l-lactic acid)

The crystallization behavior of poly(3-hydroxybutyrate) (PHB) films in blends with small amounts of poly(l-lactic acids) (PLLAs) was investigated by grazing incidence X-ray diffraction (GIXD) and infrared-reflection absorption spectroscopy (IRRAS).

Laser-Induced Periodic Surface Structures on P3HT and on Its Photovoltaic Blend with PC71BM

We describe the conditions for optimal formation of laser-induced periodic surface structures (LIPSS) over poly(3-hexylthiophene) (P3HT) spin-coated films. Optimal LIPSS on P3HT are observed within a particular range of thicknesses and laser fluences. These conditions can be translated to the photovoltaic blend formed by the 1:1 mixture of P3HT and [6,6]-phenyl C₇₁-butyric acid methyl ester (PC₇₁BM) when deposited on an indium tin oxide (ITO) electrode coated with (poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS). Solar cells formed by using either a bilayer of P3HT structured by LIPSS covered by PC₇₁BM or a bulk heterojunction with a P3HT:PC₇₁BM blend structured by LIPSS exhibit generation of electrical photocurrent under light illumination. These results suggest that LIPSS could be a compatible technology with organic photovoltaic devices.

Interfacial phenomena at a surface of individual and complex fumed nanooxides

Investigations of interfacial and temperature behaviors of nonpolar and polar adsorbates interacting with individual and complex fumed metal or metalloid oxides (FMO), initial and subjected to various treatments or chemical functionalization and compared to such porous adsorbents as silica gels, precipitated silica, mesoporous ordered silicas, filled polymeric composites, were analyzed. Complex nanooxides include core-shell nanoparticles, CSNP (50-200nm in size) with titania or alumina cores and silica or alumina shells in contrast to simple and smaller nanoparticles of individual FMO. CSNP could be destroyed under high-pressure cryogelation (HPCG) or mechanochemical activation (MCA). These treatments affect the structure of aggregates of nanoparticles and agglomerates of aggregates, resulting in their becoming more compacted. The analysis shows that complex FMO could be more sensitive to external actions than simple nanooxides such as fumed silica. Any treatment of 'soft' FMO affects the interfacial and temperature behaviors of polar and nonpolar adsorbates. Rearrangement of secondary particles and surface functionalization affects the freezing-melting point depression of adsorbates. For some adsorbates, open hysteresis loops became readily apparent in adsorption-desorption isotherms. Clustering of adsorbates bound in textural pores in aggregates of nanoparticles (i.e., voids between nanoparticles in secondary structures) causes reduced changes in enthalpy during phase transitions (freezing, fusion, evaporation). Freezing point depression and melting point elevation cause significant hysteresis freezing-melting effects for adsorbates bound to FMO in the textural pores. Relaxation phenomena for both low- and high-molecular weight adsorbates or filled polymeric composites are affected by the morphology of primary particles, structural organization of secondary particles of differently treated or functionalized FMO, content of adsorbates, co-adsorption order, and temperature.

Crystal Morphology of Poly(3-hydroxybutyrate) on Amorphous Poly(vinylphenol) Substrate

The crystalline morphology and orientation of poly(3-hydroxybutyrate) (PHB) thin film on the poly(vinylphenol) (PVPh) sublayer with different thickness was studied by atomic force microscopy, X-ray diffraction, and infrared spectroscopy. PVPh sublayer influences the morphology of PHB greatly. Although edge-on lamellae form on both Si and PVPh surfaces at relatively lower crystallization temperature, the morphology of them is quite different. It appears as sheaflike edge-on lamellar morphology on PVPh sublayer. In addition, the edge-on lamellae prefer to form on the PVPh sublayers at much higher crystallization temperature compared with that on Si wafer. The PVPh layer thickness also influences the crystalline morphology of PHB. On a 30 nm thick PVPh layer, sheaflike edge-on lamellae form in a wide range of crystallization temperatures. When the PVPh thickness increases to 65 nm, fingerlike morphology is observed when the crystallization temperature is lower than 95 °C. The fingerlike morphology is caused by a diffusion-limited aggregation process, and it requires an optimum condition. Thickness ratio between PHB and PVPh sublayer and temperature are two key factors for the formation of fingerlike morphology.

A grazing incident XRD study on the structure of poly(3-hydroxybutyrate) ultrathin films sandwiched between Si wafers and amorphous polymers

The crystallization behavior and morphology of poly(3-hydroxybutyrate) (PHB) ultrathin films sandwiched between Si wafers and amorphous thin polymer layers were studied by using grazing incident X-ray diffraction (GIXD) technology.

Are polymers glassier upon confinement?

Glass forming systems are characterized by a stability against crystallization upon heating and by the easiness with which their liquid phase can be transformed into a solid lacking of long-range order upon cooling (glass forming ability). Here, we report the thickness dependence of the thermal phase transition temperatures of poly(l-lactide acid) thin films supported onto solid substrates. The determination of the glass transition, cold crystallization and melting temperatures down to a thickness of 6 nm, permitted us to build up parameters describing glass stability and glass forming ability. We observed a strong influence of the film thickness on the latter, while the former is not affected by 1D confinement. Further experiments permitted us to highlight key structural morphology features giving insights to our ellipsometric results via a physical picture based on the changes in the free volume content in proximity of the supporting interfaces.